In recent years, it is concerned that a tsunami makes an incursion to a seacoast part of Japan due to an occurrence of a large earthquake. It is important to predict the arrival as early and accurate as possible in order to perform efficient evacuation behavior and handling before the tsunami arrives at the seacoast. Conventionally, several methods have been proposed to predict the arriving tsunami.
For example, according to a current tsunami forecast system in the Meteorological Agency, numerical simulations of tsunami are in advance performed by setting faults that possibly generates the tsunami, and the results are accumulated as a tsunami forecast database. When an earthquake actually occurs, prediction results corresponding to the location, scale and so on of the generated earthquake are retrieved from this database, and tsunami alarm or warning is announced (For example, see Patent Literature 1). However, since a location and a shift length of the fault necessary for predicting a magnitude of the generated tsunami are not accurately revealed when the earthquake occurs but subject to a data analysis at a later date, there has been a problem that large errors reside in the announced values of the magnitude, an arrival time and so on of the tsunami.
In addition, as another method, there is performed a trial to place a plurality of sensors such as buoys capable of measuring an arriving tsunami on and in the sea and to capture offshore the tsunami itself (For example, see Patent Literature 2). However, this method is able to only spatially obtain data of points, and therefore, this has been regarded insufficient to predict in detail a wave height and an arrival direction of the tsunami arriving in a wide range. Further, there has been such a problem that a large cost is necessary for securing a power source and a signal propagation path. In addition, sensors are necessary on and in the sea, and therefore, there has been such a problem that the maintenance of them are not easy.
Recently, in order to solve the aforementioned problems, tsunami monitoring with a marine radar is started (For example, see Patent Literature 3). The marine radar is able to measure ocean currents, ocean waves, ocean winds and so on in a wide region of about 100 km by radiating a radio wave from an antenna installed on a land toward the sea surface, receiving back-scattered waves due to the ocean waves at the sea surface and performing a frequency analysis. The marine radar has such a feature that a wide range can be observed at the same time, and the marine radar is suitable also for observation in a long term since observation can be performed from the land. However, the marine radar is able to measure only a sea surface flow velocity component in the line of sight of the radio wave radiated from the antenna and unable to directly measure a wave height of the arriving tsunami.
Therefore, in the tsunami monitoring system as described in Patent Literature 3, it is required to obtain a tsunami arrival time and prediction values of wave heights at nearby coasts from preliminarily prepared empirical rules of tsunami (e.g., wave height=flow velocity v×certain function F, arrival time T=distance/phase velocity and so on) on the basis of conditions of the measured flow velocity, terrain model and so on. It is required to construct in advance databases necessary for predicting tsunami characteristics from tsunami flow velocity patterns and to collate an obtained flow velocity distribution with these databases.